Rechargeable Li-ion batteries have found widespread application as a result of their low self-discharge rate, large cycle numbers and their ability to provide an indication of the remaining energy capacity.
Different applications have different requirements for the volumetric energy density, but there is always a desire to increase this, to enable smaller devices to function.
For example, for the power supply requirements of medical implanted devices, it is anticipated that the achieved volumetric energy density (Wh/L) is more important than the cost price of energy ($/Wh).
For such implantable batteries, a volume between 10 to 100 mm3 is desired.
For small batteries such as this, the volumetric energy density decreases rapidly, because the packaging takes up a larger fraction of the battery volume. An illustration of this is shown in FIG. 1, where the volumetric energy of miniaturized batteries is shown as a function of their battery volume.
One current battery considered to be at the cutting edge of technology is the Eagle Pitcher Contego 500 μAh. This is one of the smallest batteries currently known, developed for the medical implantation market. It has an energy capacity of 1.8 mWh and a volume of 30 mm3. The volumetric energy density is thus 60 Wh/L.
For medical applications, it is advantageous if the implanted device can be fully autonomous for an extended period of time (i.e. one week or longer). This will limit the risks of suffering malfunctions in the implanted devices due to a discontinuous power supply, and may also avoid the extra technical complexity of recharging efforts. If recharging efforts could be minimized, the patient could more easily leave the hospital, while the implanted device continues to do its work (e.g. for sensing & monitoring body functions).
The largest power drain for an implanted device will be caused by the need to communicate the information, as obtained by the implanted device, to the outside world and to let the outside world know that the implanted device is still operational.
For this purpose an ultra low power (ULP) radio is needed. A ULP radio typically needs a continuous power of around 70 μWatt. This means that the medical implanted device, powered by an Eagle Pitcher Contego 500 μAh battery, can only power the ULP radio for 1 day without recharging efforts. This is still typically too short for a patient to leave the hospital in a convenient manner.